it would help if you at least posted what type of RV you have.
some RV's won't have the room.

that said, having two or more batteries is only worth it if you dry camp or boondock.
if you only camp with hookups, there's no need to add a battery.
we almost always dry camp so having two batteries is a must for us, along with our Honda 2000.

It is, generally, an easy process to add a second battery. You want matched batteries - same type, same age. If you have one new one already, you can add a second one for around $100-200 depending on what brand and size you use. For a trailer with an exisitng tray on the tongue big enough to hold 2 batteries, then all you need is a plastic battery box ($10) and couple short wires to connect them. Most dealers will make the wires up for you, or you can do it yourself. There are some good reference drawings on this forum and and on some other sites for details on this. Not hard to do, but you need to use the right size wires and connect them correctly.

As Dan mentioned - this is a great thing to do if you camp without electric hookups. You essentially double your battery life.

How much help does tow vehicle battery give the battery in the trailer when it is connected? Some TV have place for second battery under hood....if that will help support the one in the trailer.....but TV would have to be connected....

Sorry Bikendan, I have a 2014 Coachmen Freedom Express 292BHDS. I have been having issues with getting my info to stay with my name. There is plenty of room for an additional battery and box if I want to add. We usually camp with hook ups if it is for an extended stay. For weekend camping we usually dry camp. Most of the time we are good with one battery but I always like to be prepared. Thus, lookig to add that additional insuarance. I have seen that they have switches that allows you to select between 1,2 or off for the batteries. I really like that option due to really giving me flexibility in the system.

How much help does tow vehicle battery give the battery in the trailer when it is connected? Some TV have place for second battery under hood....if that will help support the one in the trailer.....but TV would have to be connected....

Not as much as you would think. The construction of truck starting batteries is significantly different than a deep storage battery.

The truck battery has thin waffled surface plates to give up electrons easily in order to provide a LOT of power for a few minutes of starting; then recharge quickly. They are rated in Cold Cranking Amps (CCA).

Dual Purpose (Marine) batteries are usually installed as OEM and are a compromise between cost and performance as a house battery. They normally have their rating in CCA but also have an RC rating on it somewhere. This stands for Reserve Capacity in Minutes at a 25 amp load and is calculated differently than the Amp Hours of a Deep Storage battery which is the hours a battery can deliver a 20 amp load. To convert RC to AH for comparison, multiply RC by 0.4167 and the result is AH. So a 200 RC battery has the same battery capacity as a 83 AH storage battery.

The Deep Discharge (House) storage battery has thick solid plates with minimal surface waffling in order to pack as many electrons into the same battery volume as possible and release them at a low rate for a long time. They are rated in Amp Hours.

Since it is easier to give up electrons from the starting battery without the truck running, the trailer battery will be constantly trying to charge the truck battery.

Read the attached PDF and graph on the Peukert Effect. It shows how a storage battery's ability to deliver electrons at a given rate is reduced as that demand increases. If you ever plan on using an inverter, the high demand requires multiple batteries in order to keep the load on any one battery from exceeding the optimum discharge rate.

See the attached PDFs for more information and there is a great FAQ in that area of the forum.

Sorry Bikendan, I have a 2014 Coachmen Freedom Express 292BHDS. I have been having issues with getting my info to stay with my name. There is plenty of room for an additional battery and box if I want to add. We usually camp with hook ups if it is for an extended stay. For weekend camping we usually dry camp. Most of the time we are good with one battery but I always like to be prepared. Thus, lookig to add that additional insuarance. I have seen that they have switches that allows you to select between 1,2 or off for the batteries. I really like that option due to really giving me flexibility in the system.

well, then you need to get a Blue Seas marine battery switch and a second matching battery. the Blue Seas has the 1 or 2 battery selection.

i waited until the cheap OEM one from the dealer expired and bought two brand new matching batteries. didn't have to do any mods other than bolting two battery boxes to the tongue and adding the disconnect switch.

A second battery is a great addition if you dry camp. You do not need matching batteries if you use a switch and it gives you side benefits in charging or isolating the batteries as needed. I run two banks with 4 batteries total and have had great success with this setup but we spend a week at a time dry camping. Years ago when I was running one battery and it died while dry camping taught me a lesson in having options. Adding a second battery is a good option and I suggest the switch is the way to go to avoid a battery going bad killing your good one...